1. Field of the Invention
The present invention relates to toner for electrostatic latent image developing. In particular, the present invention relates to an electrostatic latent image-developing toner to be used for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing and the like.
2. Description of the Prior Art
Heretofore, the production of toner for use in electrophotography, electrostatic recording, electrostatic printing, and the like included the use of pulverization, suspension polymerization, and emulsion dispersion methods. According to the pulverization method, toner constituents, such as a resin, a pigment and a charge controlling agent, are mechanically kneaded and then pulverized to obtain a toner. The suspension polymerization method is used to manufacture toner by suspending a monomer, a polymerization initiator, a coloring agent and other additives in a dispersion medium, and then heating the dispersion medium to polymerize the monomer and form a resin. According to the emulsion dispersion method, an emulsion through emulsion dispersion is formed in an aqueous dispersion of a colored resin solution. The colored resin solution is prepared by dissolving a resin, a coloring agent and a charge controlling agent in a non-water soluble organic solvent. The emulsion is then simultaneously heated and stirred to evaporate the organic solvent and to precipitate fine resin particles of toner.
Of the above methods, the emulsion dispersion method is the simplest for obtaining fine resin particles averaging about 1 to 10 .mu.m in size. By using the emulsion dispersion method, greater production efficiency and lower costs are realized compared to the pulverization and suspension polymerization methods. Further, many different types of resins can be used in the emulsion dispersion method in comparison to the limited number that can be used in the suspension polymerization method. However, toner obtained by the emulsion dispersion method exhibits unstable charge performance characteristics because it is are easily affected by ambient environmental conditions, such as temperature and humidity. In addition, such toner undesirably tends to agglomerate or flocculate when stored in high-temperature environments.
After extensive research, the present inventors have discovered that trace amounts of residual organic solvent and moisture content in a toner disadvantageously affect charge performance and heat resistance characteristics of the toner. In particular, when producing toner by the emulsion dispersion method, the aqueous dispersion containing a colored resin solution is heated to evaporate and remove the organic solvent. The moisture content on the surface and interior of the resultant fine colored resin particles is conventionally eliminated by washing and by drying in a drying device. In essence, the organic solvent and moisture content are evaporated by exposing the resin particles to temperatures that are higher than the boiling point of the organic solvent and moisture content contained in the resin particles. Despite these measures, however, problems persist in that residual organic solvent and moisture content are often not adequately removed thereby resulting in trace amounts of residual organic solvent and moisture content still remaining on or near the surface of the toner. In addition, charge performance of such a toner can significantly deteriorate when it is stored in a high-temperature/high-humidity environment. Further, excess residual organic solvent content and moisture content can affect toner performance by causing particles of the toner to flocculate or agglomerate undesirably when stored at high temperatures for prolonged periods of time.